The objective of this study is to gain information on the pathogenetic mechanism of murine hepatitis caused by a member of the Bunyaviridae family, Punta Toro virus (PTV). Specifically, we are interested in the viral action upon, and the interaction between hepatocytes, Kupffer cells, endothelial cells, T-lymphocytes and interferon during initial stages of the development of PTV-hepatitis. A striking age-dependent resistance to PTV is acquired in mice which cannot be entirely explained by humoral immunity, cell-mediated immunity or interferon production alone. Since the course of PTV disease appears to be determined in the first 24 hrs after infection, it is likely that resistance involves the first natural line of defense to viral infection and replication, the host-target cell. It is proposed that the mature Kupffer cell is such a key determinant of resistance against early viral hepatocellular injury and death in PTV infection, as it is in other viral causes of murine hepatitis. The interaction of hepatocytes and Kupffer cells, T-lymphocytes, and interferon are probable factors which influence the host genetic predisposition of this cell type. At present, the Kupffer cell is thought to play an insignificant role in hepatic injury due to Bunyaviridae. In this study, the pathogenesis of Punta Toro viral infection will be studied in vivo, and in vitro with particular emphasis on the role of the Kupffer cell in the murine hepatitis caused by this virus. Two groups of C57BL/J6 mice will be used, one group of 4 week old mice extremely susceptible to PTV hepatitis, and another group of 8 week old mice resistant to PTV (see Background, preliminary results). The effect of Punta Toro virus infection upon the cells of the liver in vivo will be examined by correlation of viral replication and viremia with sequential serum levels of liver transaminase enzyme, histopathological effects, and cellular localization of viral antigens at the light microscopy and ultrastructural level. In vitro, Punta Toro viral infection of liver-derived cells will be examined by the presence of viral proteins, evidence of replication and ultrastructural changes in isolated hepatocytes, endothelial cells, and Kupffer cells; and in co-cultures of hepatocytes with Kupffer cells, or the supernatants of Kupffer cells. The modulating role of interferon and thymus-derived lymphocytes will also be studied in vivo and in vitro.